Electromagnetic couplers, such as radio frequency (RF) couplers, are used in a variety of applications to extract a signal for measurement, monitoring, or other uses. For example, an RF coupler can be included in a signal path between an RF source and a load (such as an antenna) to provide an indication of forward RF power of the RF signal traveling from the RF source to the load and/or an indication of reverse RF power reflected back from the load. An RF coupler typically has a coupled port, an isolation port, a power input port, and a power output port. When a termination impedance is presented to the isolation port, an indication of forward RF power traveling from the power input port to the power output port is provided at the coupled port. When a termination impedance is presented to the coupled port, an indication of reverse RF power traveling from the power output port to the power input port is provided at the isolation port. The termination impedance is typically implemented by a 50 Ohm shunt resistor in a variety of conventional RF couplers.
FIG. 1 is a block diagram illustrating an example of a typical arrangement of an RF “front-end” sub-system or module (FEM) 100 as may be used in a communications device, such as a mobile phone, for example, to transmit RF signals. A power amplifier module 110 provides gain to an RF signal 105 input to the FEM 100, producing an amplified RF signal. The power amplifier module 110 can include one or more power amplifiers. The FEM 100 further includes a filtering sub-subsystem or module 120, which can include one or more filters in each of the transmit path (TX) and receive path (RX). In particular, in the transmit path the filter sub-system 120 is used to filter out unwanted frequencies from the amplified RF signal, but includes unwanted resistive loss in-band. A directional coupler 130 is used to extract a portion of the power from the RF signal traveling between the power amplifier module 110 and an antenna 140 connected to the FEM 100. The antenna 140 transmits the RF signal, and can receive RF signals. A switching circuit 150, also referred to as an antenna switch module (ASM), can be used to switch between a transmitting mode and receiving mode of the FEM 100, for example, or between different transmit or receive frequency bands. The switching circuit 150 can be operated under the control of a controller 160.
Referring to FIG. 2 there is illustrated a schematic representation of one example of the directional coupler 130 (also referred to as an RF coupler). The directional coupler 130 has a power input port 202 that receives the RF signal (e.g., from the power amplifier module 110 or filter sub-system 120), a transmitted port 204 (also referred to as a power output port) at which the RF signal is provided for transmission by the antenna 140, a coupled port 206, and an isolation port 208. The directional coupler 130 includes a main transmission line 210 that extends between the input port 202 and the transmitted port 204, and a coupled transmission line 212 that extends between the coupled port 206 and the isolation port 208. A termination impedance 214 is typically connected to the isolation port 208. The directional coupler 130 has a coupling factor, which represents how much power is provided to the coupled port 206 of the coupler relative to the power of the RF signal at the power input port 202. Directional/RF couplers typically cause an insertion loss in the RF signal path between the power amplifier module 110 and the antenna 140, in part due to the coupling factor. Thus, an RF signal received at the power input port 202 of the directional coupler 130 generally has a lower power when provided at the transmitted port 204 because some of the signal power has been coupled off and provided at the coupled port 206. Insertion loss can also be affected by losses associated with the main transmission line 210 of the directional coupler 130.